Primary immunization of different inbred strains of mice with the house dust mite allergen Der p I (from Dermatophagoides pteronyssinus) adsorbed to adjuvant revealed differences in IgE but not IgG responses. Thus, the CBA and C57 Black strains were shown to be high IgE responders in that persistent IgE was induced which lasted for several months. In contrast, the C3H, AKR and Balb/c strains were judged to be poor responders. Subsequent experiments showed that such responses were adjuvant-independent, refractory to irradiation (850 rad) and that they could be adoptively transferred using immune spleen cells in a dose-dependent manner. At sub-optimal doses, adoptively transferred IgE responses were initiated by either intraperitoneal or intranasal challenge of recipients with allergen in the absence of adjuvant. Allergen-specific IgE in rats was also shown to be strain-dependent, with Brown Norway rats, in contrast to Wistar Furth (WF) and Lou/M rats, being the highest responder strain studied. Repeated intranasal administration of soluble allergen induced IgE tolerance in rats but not mice. However, tolerance was restricted to the low IgE responder phenotype strain, WF.